1. Technical Field
The present invention relates in general to magnetic data storage and retrieval systems and in particular to methods and apparatus for predicting head-disk interactions in magnetic disk storage and retrieval systems. Still more particularly, the present invention relates to an improved method and apparatus for predicting head-disk interactions (e.g. head crashes) in magnetic disk storage and retrieval systems on the basis of sway mode frequency presence in a position error signal.
2. Description of the Related Art
Generally, a data access and storage system consists of one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic data storage and retrieval system is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The disks themselves in a hard disk drive (HDD) are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, two or more disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm).
The only other moving part within a typical magnetic data storage and retrieval system is the actuator assembly. Within most magnetic data storage and retrieval systems, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk.
Typically, a slider is formed with an aerodynamic pattern of protrusions (air bearing design) on its air bearing surface (ABS) that enables the slider to fly at a constant height close to the disk during operation of the disk drive. A slider is associated with each side of each disk platter and flies just over the platter's surface. Each slider is mounted on a suspension to form a head gimbal assembly (HGA). The HGA is then attached to a flexible suspension, which is attached to a rigid arm. Several arms are ganged together to form a head/suspension/arm assembly.
Each read/write head scans the surface of a disk during a “read” or “write” operation. The head/suspension/arm assembly is moved utilizing an actuator that is often a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting is in turn mounted to a frame via a compliant suspension. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop directly over the desired track.
In normal operation, the slider and head fly over the surface of the disk at a vertical height on the order of 2 millionths of an inch. The microscopic distance between the recording surface and the read/write head leaves little tolerance for vertical misalignment. Even very small angular misalignments of components resulting from wear, mismanufacture, or foreign objects on the surface of the disk can cause the head to come in contact with the recording surface. Such physical contact may cause the slider to “fishtail” temporarily in the plane of the disk surface. Repeated physical contacts in the same location on the disk surface may lead to a head-disk crash. This renders the disk inoperable and destroys any data stored on the recording surface.
Computer users have traditionally, if bitterly, accepted as inevitable the random loss of data due to a head-disk crash. Frequent backups limit the magnitude of data loss, but no convenient and cost-effective solution exists for entirely preventing the loss of data. Greater backup frequency reduces the magnitude of the loss but increases the magnitude of the inconvenience to the user. Redundant storage solutions reduce data loss but degrade system performance and increase system cost. Users have long desired, and industry has unsuccessfully attempted to produce, a warning that would inform users of an impending head-disk crash. With a proper warning of an impending head-disk crash, users could perform an immediate backup of desired data and thereby completely eliminate data loss from head-disk crash events.